Drill bodies for cutting or chip removing machining may have the most varying design to be able to carry out efficient machining operations in workpieces of metal of varying type. Usual materials in which hole machining is effected are steel, cast iron, aluminium, titanium, stainless materials, as well as different types of composites. A frequently occurring machining method is solid drilling, which means that a hole is drilled in a solid material at a predetermined diameter in a single operation, while forming either a through hole having two openings or a blind hole having a bottom and only one opening. Such drilling may be carried out by means of short hole drills, the length of which usually amounts to at most 5×D, as well as long hole drills, the length of which may amount to 100×D or more. Another method is so-called reaming, wherein a pre-drilled pilot or center hole is imparted with the final hole diameter and optimum quality in respect of dimensional accuracy and surface smoothness.
While short hole drills usually have the cutting inserts and cutting edges thereof symmetrically placed and therefore are self-centering, long hole drills, above all of the type STS and EJECTOR that require internal chip evacuation, are made with asymmetrically placed cutting inserts, which makes self-centering impossible. More precisely, such drill bodies are, in the front end thereof, equipped with three cutting inserts, viz. a peripheral insert, a center insert and an intermediate insert, the two first-mentioned ones of which are placed adjacent to a first chip inlet and the last-mentioned one to a diametrically opposite, second chip inlet. The cutting inserts can be fixed either by soldering or by means of screw joints. Behind the two chip inlets, as viewed in the direction of rotation, support pads are mounted in the envelope surface of the drill body and project radially so much from the envelope surface that the radially outermost portion thereof will abut against the hole wall generated by the peripheral cutting insert, all with the purpose of centering the drill body during the axial feed thereof. Examples of long hole drills having such support pads are disclosed in U.S. Pat. No. 5,697,737, U.S. Pat. No. 6,602,028 B1 and U.S. Pat. No. 6,682,275 B1.
Also other drill bodies, e.g., for boring purposes, make use of support pads to center the drill body, such as are exemplified in U.S. Pat. No. 4,571,130, U.S. Pat. No. 4,596,498 and U.S. Pat. No. 5,551,812.
Generally, individual support pads for the above-mentioned purposes have a parallelepipedic (usually elongate) basic shape and consist of a hard-wearing material, such as cemented carbide, the support pads being delimited by a cross-sectionally convexly arched outer surface for abutment against the hole wall, an inner surface for pressing against a bottom of a mounting pocket in the drill body, front and rear end surfaces, as well as a pair of mutually parallel side surfaces. The outer surface, which also may be denominated contact surface, is generally cylindrical, although the same also may have an utmost slight conicity in the backward direction. To facilitate the entering of the support pad into the hole that has been initiated by those cutting inserts that enter the workpiece immediately before the support pad, between the outer surface and at least the front end surface, it is formed a so-called entering surface in the form of a conical surface that tapers in the forward direction. In addition, support pads of drill bodies for the drilling of through holes may be formed with such an entering surface also at the rear end thereof, more precisely to facilitate the return of the drill body through the hole. Furthermore, it should be mentioned that the radius of curvature of the convexly arched outer surface of most support pads is selected somewhat smaller than the radius of the hole that should be recessed, the contact between the hole wall and the support pad getting the form of an axial line contact somewhere along the outer surface. In such a way, the support pad may be used for drill bodies having different diameters contrary to support pads, the outer surface of which have a radius of curvature that coincides with the hole radius.
The present invention has its origin in problems that are associated with previously known support pads and the drill bodies to which these are applied. Thus, the shape of the entering surface of previously known support pads is geometrically determined by a truncated cone, the basal plane of which has the same radius as the radius of curvature of the cylindrical outer surface, at the same time as the cone and the cylinder are coaxial, i.e., generated along a common center axis to which the basal plane of the cone (and one end plane of the imaginary cylinder) is perpendicular. This means that the front and rear borderlines, which delimit the entering surface, become parallel to each other as viewed in planar view toward the outer surface of the support pad, at the same time as the two end borderlines of the entering surface form generatrices of the truncated cone. In practice, said geometry entails a plurality of disadvantages, one of which consists of the fact that the contact spot or contact line of the outer surface against the hole wall cannot be controlled or predetermined. When the rear borderline of the entering surface enters into the initiated hole, the contact point may accordingly end up on different spots along the rear borderline; something that may result in an impaired centering of the drill body and a mediocre hole quality in respect of dimensional accuracy and surface smoothness. Another disadvantage is that the two end borderlines of the entering surface form fairly sharp edges, which during the rotation of the drill body move in the same cone-shaped path of motion as the other generatrices along the entering surface. Therefore, the sharp edge that first (depending on the direction of rotation of the drill body) impinges on the hole edge may easily come to scrape against or dig into the same.
Most suitably, the outer surface as well as the entering surface obtain the final shape thereof by grinding when the support pad has been mounted on the drill body. To avoid the negative effects that the sharp end edges of the entering surface cause, it happens that these edges are finished. Such finishing may even be effected in a manual way, e.g., by honing or edge smoothing. However, such finishing makes the manufacture of the support pad, which is a mass-produced article that requires efficient series manufacturing to create economy, considerably dearer.